The copolymer of 1-(acetylsalicylyloxy) ethyl methacrylate (HEMA-ASA) with me-thacrylic acid (MA) has been prepared and the hydrolysis of this copolymer in dio-xane-H2O either in acidic or in alkaline medium

at 60℃ or 37℃ have been investigated. It was found that the chief product of hydrolysis is always aspirin with minor amount of salicylic acid. Due to its neighbouring effect

the hydrolysis rate of (HEMA-ASA)-MA copolymer markedly depends on its composition and the pH value of hydrolysis medium

i.e. the hydrolysis rate increases with increasing the mole frac-tion of MA in copolymer and the pH value of hydrolysis medium.